The proposed research: (1) investigates how human mothers, through nursing, influence the behavior and physiological states of their infants from birth until 12 weeks of age, and (2) analyzes peripheral and central mechanisms that mediate these changes. Specific Aim I determines the qualitative and quantitative effects of milk and its components, protein, fat and lactose, alone and in combination, on calming, energy-conservation and on coping with pain. It also evaluates whether opioid processes contribute to these changes by studying infants born to women who were maintained on methadone during pregnancy. Specific Aim II extends these investigations to infants 2-12 weeks of age. We have already discovered that, unlike newborns, who are quieted by as little as 0.1 ml. of an effective tastant, 2-week-old infants must receive a minimum of 0.5 ml. once/min. Dose-response functions will determine whether this change is qualitative or quantitative only. We have also discovered that infants 6 weeks of age and older are quieted only when effective solutions are presented to them when they are visually engaged by the individual delivering the solution. We will determine whether the taste- (or olfaction-) induced mechanism has undergone additional qualitative or quantitative changes in these older infants. The crucial aspect of visual contact that allows the taste (olfactory) driven mechanism to calm and induce energy-conservation will be identified. Specifically, we will determine whether visual stimulation is effective through eye contact per se, through seeing the investigator's face, or through the contingent movement of the experimenter in maintaining visual contact. Finally, in order to determine the motivational consequences of these state changes, infant visual preferences or aversions for the individual delivery sucrose, milk, or water will be assessed. Taken together, these studies qualitatively and quantitatively document the psychobiology of human affect during early infancy and may start to provide a biological basis for the emotional foundations of the mother-infant relationship.